1. Field of the Invention
This invention relates to rolling mills, and is concerned in particular with an improvement in the apparatus and methods employed to subject hot rolled steel rod to controlled cooling in order to achieve optimum metallurgical properties.
2. Description of the Prior Art
In a conventional rolling mill installation, as depicted in FIG. 1, hot rolled steel rod 10 emerges from the last roll stand 12 of the mill at a temperature of about 750.degree.-1100.degree. C. The rod is then rapidly water-quenched down to about 550.degree.-1000.degree. C. in a series of water boxes 14 before being directed by driven pinch rolls 16 to a laying head 18. The laying head forms the rod into a continuous series of rings 20 which are deposited on a cooling conveyor generally indicated at 22. The conveyor has driven table rollers 24 which carry the rings in a non-concentric overlapping pattern through one or more cooling zones. The conveyor has a deck 26 underlying the rollers 24. The deck is interrupted by slots or nozzles 28 through which a gaseous cooling medium, typically ambient air, is directed upwardly between the rollers 24 and through the rings being transported thereon. The cooling air is driven by fans 30 connected to the nozzles 28 via plenum chambers 32. The thus cooled rings drop from the delivery end of the conveyor into a reforming chamber 34 where they are gathered into upstanding coils.
As can best be seen in FIG. 2, the non-concentric overlapping ring pattern has a greater density along edge regions 36 of the conveyor as compared to the density at a central region 38 of the conveyor. Therefore, a greater amount of air is directed to the edge regions 36 of the conveyor to compensate for the greater density of metal at those regions. Typically, this is achieved by increasing the nozzle or slot area at the edge regions. As illustrated in FIG. 2, this can be accomplished by locating short slots or nozzles 28a at the edge regions 36 between longer slots or nozzles 28b which extend across the full conveyor width. Alternatively, full width nozzles or slots may be employed exclusively in conjunction with mechanical means such as vanes, dampers, etc. (not shown) in the plenum chambers to direct more air to the conveyor edge regions 36.
The cooling path through metallurgical transformation is a function of the air velocity and the amount of air (among other factors) applied to the rod. Thus, as the rod is conveyed by the table rollers 24 over successive mutually spaced slots or nozzles 28, the resulting intervals between coolant applications produce a stepped cooling path as shown in FIG. 3.
As shown in FIG. 4, with a greater number of coolant applications at the edge regions 36 as compared to the central region 38, the non uniform intervals between successive coolant applications will result in one cooling path P.sub.36 at the edge regions 36 and a different cooling path P.sub.38 at the central region 38. These different cooling paths cause different rod segments to pass through transformation at different temperatures and at different rates, resulting in non-uniform metallurgical properties along the length the rod.
A related disadvantage of conventional air distribution systems is the "hard" transition from high air velocities at the conveyor edge regions 36 to lower air velocities at the central region 38. Where different numbers of nozzles are located at the edge and central conveyor regions as illustrated in FIG. 2, the edge nozzles 28a supply air only over a discrete portion of the total width of the steel rings being cooled. There is a sudden change from intense air cooling to no air cooling at the transition between the edge and the central regions. In the case of nozzles which span the entire width of the conveyor as used in conjunction with vanes or dampers to direct more flow to the edges, there is also a "hard" transition from high flow at the edges to lower flow in the center. This is a result of the presence of dividers in the plenum chamber upstream of the nozzles, which channel the air from the fans to the nozzles.
The objective of the present invention is to avoid the above-described drawbacks of conventional air distribution systems by applying cooling air to all ring segments at regularly spaced intervals, coupled with a decrease in the air flow rate at the central region of the conveyor, where ring density is lower than that at the conveyor edge regions.
A companion objective of the present invention is the elimination of hard transitions from high air velocities at the conveyor edge regions to low air velocities at the conveyor central region.